These studies are to determine the effects of varying levels of positive end-expiratory pressure (PEEP) and high frequency ventilation (HFV) on pulmonary transvascular fluid exchange in the normal and high permeability lung. Studies will be made in the intact, anesthetized sheep in which it is possible to collect pure lung lymph. Determination of lymph flow of lymph and plasma protein concentration and wet-to-bloodless dry lung ration will be used to assess possible changes in fluid filtration and vascular permeability as they may or may not occur as a result of PEEP or HFV in the normal and vascular injured lung. In addition, the overall and regional pulmonary blood flow with labelled microspheres will be studied under the various experimental conditions and different modes of ventilation. These studies will be correlated with measurement and interstitial pressure according to the method of Parker et al. and may provide physiological bases for the mechanism by which PEEP and HFV may have their effects on pulmonary transvascular fluid balance. HFV is a clinically very promising new mode of ventilation that may find wide application in the intensive care setting. Since its physiological base is not known, pulmonary hemodynamics and overall gas exchange in the normal and high permeability lung of the dog will be performed utilizing a large field scintillation camera and labelled microspheres. With the HFV experiments the mean airway pressure will be kept minimal (203 cm H2O), as this is a major advantage of HFV over PEEP ventilation.